1. Field of the Invention
The present invention relates to an organic electroluminescence element. In particular, the invention relates to an organic electroluminescence element with high light-emission efficiency and excellent durability.
2. Description of the Related Art
An organic electroluminescence element (hereinafter, referred to as an “organic EL element” in some cases) is composed of a light-emitting layer or a plurality of organic functional layers containing a light-emitting layer, and a pair of electrodes sandwiching these layers. The organic EL clement is a device for obtaining luminescence by utilizing at least either one of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a hole injected from an anode to produce the exciton in the light-emitting layer, or luminescence from excitons of other molecules produced by energy transmission from at least one of the above-described excitons.
Heretofore, an organic EL element has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and device efficiency are remarkably improved. For example, “Science”, vol. 267, No. 3, page 1332, (1995) discloses a two-layer laminated type device obtained by laminating a hole transport layer and a light-emitting layer also functioning as an electron transport layer; a three-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, and an electron transport layer; and a four-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, a hole blocking layer, and an electron transport layer.
However, many problems still remain for putting organic EL elements to practical use. First, there is a need to attain high light-emission efficiency, and second, there is a need to attain high drive durability. In particular, deterioration in quality during continuous driving is a most significant problem.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2003-123984 discloses an attempt to dispose an interface layer of 0.1 nm to 5 nm as a barrier layer between a light-emitting layer and a hole transport layer and retard the migration of holes, to thereby control the migration balance between holes and electrons and enhance the external quantum efficiency. However, this means potentially involves a problem of lowering the brightness and increasing the driving voltage since the migration of all of the carriers is lowered, as well as a problem of lowering the drive durability, since the time that the-carriers stay in the device is made longer.
Further, a configuration in which light emitting units each containing a light-emitting layer and a functional layer are stacked in a multi-layer structure (which is referred to as “multi-photon”) is known. For example, JP-A No. 6-310275 discloses a configuration in which plural light-emitting units including an organic electroluminescence element are isolated by an insulating layer, and opposing electrodes are provided for each of the light emitting units. However, in this configuration, since the insulating layer and the electrode between the light emitting units hinder the extraction of emitted light, the light emitted from each of the light-emitting units cannot substantially be utilized sufficiently. Further, this is not a means for improving the low external quantum efficiency inherent to each of the light emitting units.
In the case of a polymer dispersion type light-emitting element, since a light-emitting layer generally has a monolayer configuration, light-emitting sites are dispersed in the light-emitting layer. Therefore, there has been a problem in that it is difficult to achieve a balance between injection and transport of a hole and an electron, whereby the recombination efficiency is reduced. As a means for improving this problem, JP-A No. 2001-189193 discloses that in a light-emitting layer, both the concentration of a light-emitting material and the concentration of a charge transport material are made low on the anode side and high on the cathode side, respectively, thereby causing light emission concentratedly in a region on the cathode side. Though this means is effective against peculiar problems in the polymer dispersion type light-emitting element, the light-emitting region is located in only a part of the region on the cathode side, and the whole of the light-emitting layer is not effectively utilized. Therefore, it may be said that this is not an overall enhancement of the light-emission efficiency.
Also, in the case where an organic EL element is of a laminate structure, carrier injectability is reduced due to a barrier between the respective layers, and there are problems of an increase in driving voltage and a reduction in durability. As a means for reducing such a barrier between the respective layers, JP-A No. 2002-313583 proposes to provide a gradation in the concentration of a hole injection material, an electron injection material, a hole transport material or an electron transport material, contained in the respective layers. In such a configuration, a light-emitting material in a light-emitting layer is disposed in a restricted region in the light-emitting layer which is formed of a bipolar mixed layer. Even in this configuration, light emission takes place only in the restricted region where the light-emitting material is disposed.
In designing a practically useful organic EL element, it is an extremely important problem to make high external quantum efficiency and high drive durability compatible with each other. This problem is a problem which is always required to be improved.